Amplifiers are essential components in a wide variety of systems, e.g. electronics systems, such as, for example, communications systems, radar systems, TV systems, etc. Amplifiers can however also be used in a variety of systems other than electronics systems, e.g. acoustics, optics etc.
When amplifiers are used in communications systems, increasing demands for higher data rates and greater bandwidths impose severe challenges for designing highly efficient amplifiers in order to avoid amplifiers which exhibit inherent non-linearities in their transfer functions. Nonlinearities in amplifiers in communications systems introduce at least two problems: spectral growth beyond the signal bandwidth, which interferes with adjacent channels, and distortion within the allocated bandwidth, which results in increased bit error rates.
In order to linearize transmitter amplifiers, Digital Signal Processing, DSP, techniques are often employed to compensate for nonlinear distortions and to achieve highly efficient and reasonably linear amplifiers. Among others, digital pre-distortion, DPD, is a cost effective technique which is widely used in transmitters to provide a linear signal at the output of transmitter PAs.
Usually, in communications systems, efforts to correct for amplifier non-linearities, i.e. to “linearize” amplifiers, have taken place in transmitters, whereas efforts to linearize amplifiers in receivers have been less common.
Naturally, receiver amplifiers could conceivably be linearized by means of techniques similar to those used in transmitter amplifiers, e.g. DPD, but this would, however, require the distorted signal after the post-distorter to be compared with a reference signal before the amplifier, thereby necessitating complex implementations including, inter alia, analogue to digital conversion, compensation for gain imbalances, compensation for DC offsets, power adjustment. Using post-distortion in order to linearize a receiver amplifier would also pose stringent requirements on time alignment in the comparisons.